1. Field of the Invention
The invention relates to a method for determining the optimal excitation value (stator current, magnetic flux, magnetic induction) which minimizes the electromagnetic (copper and iron) losses of DC machines. The invention provides the command to the excitation control loop of the DC machines.
2. Description of the Prior Art
It is known that, when a DC machine is not required to operate at its nominal load, it need not be excited with its nominal excitation. See M. Kostenko, L. Piotrovsky: "Machines Electriques", pp. 268-272, Ed. Mir, Moscow, 1974; and C. C. Siskind: "Electrical Machines Direct and Alternating Current", pp. 191-194, McGraw-Hill, 1959. All excess excitation contributes to an increase of the machine's electromagnetic losses. By adjusting the excitation of the DC machine according to its load requirements, its electromagnetic losses are decreased and its efficiency is accordingly increased. It is obvious that, the lower the load, the higher the increase in efficiency. When the load equals its nominal value, the nominal excitation is then required. In that case the efficiency of the DC machine cannot be increased, unless errors were made during the design of the machine, or inadequate materials were used during manufacturing. It is known that, at steady state (when both speed and torque are constant) the electromagnetic losses of DC machines can be minimized if the excitation is accordingly adjusted. The minimality condition comes in the following general form: EQU x.sub.opt =f(i.sub.a, .omega., .alpha.) (1)
where i.sub.a is the rotor current, .omega. is the speed, .alpha. are the parameters of the DC machine and of its loss model and x.sub.opt is the optimal excitation. See Kostenko, Piotrovsky, cited above; A. Kusko, D. Galler: "Control Means for Minimization of Losses in AC and DC Motor Drives", I.E.E.E. Trans. on Industrial Applications, Vol. 19, No. 4, July-Aug. 1983; S. C. Hong, M. H. Park: "Microprocessor-Based Optimal Efficiency Drive of Separately Excited DC Motor", Proc. of I.E.E.E. TEN CON, pp. 126-128, 1984; T. Egami, J. Wang, T. Tsuchiya: "Efficiency-Optimized Speed Control System Synthesis Method Based on Improved Optimal Regulator Theory--Application to Separately Excited DC Motor System", I.E.E.E. Trans. on Industrial Electronics, Vol. 32, No. 4, pp. 372-380, Nov. 1985; T. Egami, T. Tsuchiya: "Efficiency-Optimized Speed Control System Based on Improved Optimal Regulator Theory, I.E.E.E. Trans. on Industrial Electronics, Vol. 33, No. 2, pp. 114-125, May 1986; and S. C. Hong, M. H. Park: " Microprocessor-Based High-Efficiency Drive of DC Motor", I.E.E.E. Trans. on Industrial Electronics, Vol. 34, No. 4, pp. 443-440, Nov. 1987. Eq. (1) is valid for any DC machine, regardless of sort of the excitation (separate, compound or series). Eq. (1) is equally valid for ac machines which are controlled using the vector control method. When the vector control method is applied, the AC machine behaves exactly as a separately excited DC machine. See B. K. Bose: "Power Electronics and AC Drives", pp. 264-276, Prentice-Hall, 1986; and G. K. Dubey: "Power Semiconductor Controlled Devices", p. 57, Prentice-Hall, 1989. From Eq. (1) it is obvious that, if the parameters of the DC machine and of its loss model are known, the optimal excitation can be determined by measuring its speed and its rotor current. The same applies for vector-controlled ac machines. The specific form of Eq. (1) heavily depends upon the assumed loss model and the magnetization curve of the DC machine. In general, the form of Eq. (1) is complex and cannot be implemented with analog electronic circuits. See A. Kusko, D. Galler; S. C. Hong, M. H. Park (1984); T. Egami, J. Wang, T. Tsuchiya; and T. Egami, T. Tsuchiya (1986), all of which cited hereinabove. To date Eq. (1) is implemented using microprocessors. See A. Kusko, D. Galler; S. C. Hong, M. H. Park (1984); T. Egami, J. Wang, T. Tsuchiya; T. Egami, T. Tsuchiya (1986); and S. C. Hong, M. H. Park (1987), all of which cited hereinabove; as well as T. Egami, T. Tsuchiya: "Efficiency-Optimized Speed Control System with Feed-Forward Compensation", I.E.E.E. Trans. on Industrial Electronics, Vol. 34, pp. 216-226, May 1987. The microprocessor-based implementation of Eq. (1) generally following two directions. The first focuses on the direct calculation of the second part of Eq. (1). See S. C. Hong, M. H. Park (1984); T. Egami, J. Wang, T. Tsuchiya; T. Egami, T. Tsuchiya (1986); T. Egami, T. Tsuchiya (1987); and S. C. Hong, M. H. Park (1987), all of which cited hereinabove. The second relates to the creation of a lookup table and the seeking of the optimal excitation for every value of the speed and the rotor current of the DC machine. See S. C. Hong, M. H. Park (1987), cited hereinabove.
The microprocessor-based implementation of Eq. (1) and the resulting control system present some serious deficiencies, the most important of them being:
1. The microprocessor-based implementation for old-technology DC machines that use analog control techniques is not simple and comes at a high cost. PA1 2. The microprocessor-based implementation of Eq. (1) requires knowledge of the machine's loss model. Measuring the loss model parameters is extremely difficult and time comsuming, especially for machines already installed and operating. Besides, the loss model parameters of machines coming out of the same production line might be quite diverse, as are the magnetic properties of the iron used for their manufacturing. PA1 3. The direct calculation of the second part of Eq. (1) is complex and time consuming. Thus, a considerable delay is encountered at the control system and the DC machine is not sufficiently responding to abrupt load variations. PA1 4. The direct calculation of the second part of Eq. (1) leads to the minimization of losses, only of the DC machine and not of the losses of the whole control system. PA1 5. Creating a lookup table requires the execution of a variety of measurements. These measurements must be executed for every single machine and are time consuming. For machines already in operation such measurements are practically impossible. Large amounts of memory are also required for storing all possible combinations of i.sub.a, .omega. and x.sub.opt. As storing every single combination of i.sub.a, .omega. and x.sub.opt is not feasible, minimal losses are not always achieved. PA1 1. Simple application of the method. PA1 2. Very low cost and simple construction of the electronic device. PA1 3. Simple application of the method and low-cost installation of the electronic device in old-technology already operating DC machine systems, that use analog control techniques. PA1 4. Simple application of the method or low-cost installation of the electronic device in modern-technology DC machine systems, that use digital control techniques (microprocessors). PA1 5. Very fast response to abrupt load variations and absence of effects on the DC machine dynamics. PA1 6. Minimization of losses, not only of the DC machine but of the losses of the whole control system as well. PA1 7. No knowledge of the machine's loss model is required. PA1 8. Adjusting the parameters of the invention is very simple; only very few measurements are needed.
It is widely recognized that all existing minimum loss control methods are complex, come at a high cost and are, therefore, employed only when the energy saving is large enough to justify the additional cost and complexity. See G. K. Dubey, cited hereinabove.